The JVC HM-200E is the first example (of which I'm aware) where microphone elements were mounted in a headphone case.
I've copied the advertisement below from a brochure I got in the mid 70's for JVCs portable cassette boomboxes. Some JVC boomboxes
(the RC-828 for example) had a unique built-in "Biphonic Processor" that allowed a listener seated 80 cm from the stereo to hear
binaural recordings correctly.

David Simpkins sent me a jpeg that shows an interesting aspect of the design of these phones.
The mic elements actually point "backwards".
David pointed out that the housing is shaped to imitate the ear, with the capsules receiving a proportion of the
sound from reflections off the mouldings.